Tape the cord of her phone closest to the receiver to the end closest the phone itself. When they go to pick up the phone it will all come flying with it. It might sound stupid but it has a huge comedic value.

Another one is to put just a tiny bit of Aloe gel on their ear piece on their phone. I had some stuff that was unscented so people would all be WTF is this shit. Drive them nuts when they would get a glop of goo in their ear and not know what it was.